Field
The present disclosure relates to detectors of radio-frequency (RF) signals.
Description of the Related Art
In many radio-frequency (RF) applications it is advantageous to be able to detect the power level of a signal. An approach is to detect the envelope of a signal or its peak voltage.
For example, in some applications it is desirable to know the voltage level of a signal outputted from a power amplifier (PA) to provide feedback for controlling the outputted signal level. This may be incorporated into automatic gain control, wherein a feedback control loop adjusts the gain of an amplifier based on the input signal power level so the output signal power level is relatively constant.
An approach proposed by Meyer, R. G. in “Low-power monolithic RF peak detector analysis,” IEEE Journal of Solid-State Circuits, vol. 30, no. 1, pp. 65,67, January 1995, the entire contents being hereby incorporated by reference, makes use of a first bipolar transistor as rectifying element and a second bipolar transistor to set up an offsetting dc voltage. A circuit according to this approach is shown in FIG. 1. Meyer presents an analysis for large square-wave input signals that relates the detected voltage to the input signal peak voltage plus an error term represented by VT·In(2), in which VT is the thermal voltage given by kBT/q, in which q is the magnitude of the electrical charge of an electron, kB is the Boltzmann constant and T is the absolute temperature, VT having a value of approximately 25 mV at room temperature. It is further disclosed that the VT-dependent error term can be cancelled by doubling the current in the second transistor relative to the first transistor. As the derived error term is only an approximation to the true error in a detector, practical implementations of this approach do not provide an exact measurement of the voltage level of the input signal, particularly for low voltage levels, having a non-linear response to the voltage level of the input signal.
Therefore it would be desirable to provide a detector with improved accuracy and/or linearity. It would further be desirable to provide such a detector without greatly increasing the complexity, size or cost of the detector.